This application relates to the removal of certain metals from liquid hydrocarbons. More particularly, it relates to the removal of metals from liquefied hydrocarbons, often referred to as LPG (liquefied petroleum gas), that is, a stream containing hydrocarbons normally gaseous at ambient temperature but which has been liquefied for storage and transportation convenience. The metals whose removal is specifically addressed are mercury, lead, antimony, arsenic, and copper.
LPG often is a byproduct of liquefied natural gas (LNG) production, and contains mainly C.sub.3, C.sub.4, and C.sub.5 hydrocarbons. Several metals, principally mercury and to a lesser but nonetheless important extent antimony, lead, arsenic, and copper are natural contaminants as LPG is produced from wellhead gas. Although metal concentrations rarely attain levels even as high as 1 ppm, nonetheless their presence even at concentrations above 1 ppb can be deleterious. For example, mercury as a chief offender at levels of even several ppb causes corrosion of aluminum, which is extensively used in the heat exchangers of the separation-adsorption train. Arsenic is a poison whose removal is required for health and safety reasons. Consequently there is a need for the removal of these metals from LPG as well as from other liquid hydrocarbon feedstocks.
Present methods of removal are based on the adsorption of the subject metals by suitable materials. Among the adsorbents currently used may be mentioned charcoal, especially charcoal containing sulfur (U.S. Pat. No. 3,194,629) and other mercury-reactive materials (U.S. Pat. Nos. 4,196,173 and 4,500,327), ion-exchange adsorbents having thiol groups (U.S. Pat. No. 4,950,408) and polysulfides (U.S. Pat. No. 4,591,490), and certain silver oxide impregnated zeolites. All have limitations based on their capacity, the degree of metal removal at equilibrium, their regenerability, or some combination of the above. In this application we disclose a combination of adsorbents which, when used in series, is exceptionally effective in removing the aforementioned metals, and especially mercury, to leave levels of no more than about 1 ppb (parts per billion), and generally no more than 0.1 ppb, in the treated LPG. In another aspect our invention is a method of determining the concentration of metals of the aforementioned class in liquefied hydrocarbons at concentrations down to at least 1 ppb.